As a continuation of a general investigation of trace element deposition in ambient aerosol inhalation using techniques compatible with highly sensitive elemental analysis by the the method of particle induced X-ray emmission, PIXE, the effect on pulmonary function of human subjects of exposure to low concentrations of gaseous SO2 and fine aqueous aerosol droplets will be investigated for a population of 15 selected individuals using the methods of Johnston, Winchester, nd Wasson (1981). Using these methods and a single healthy subject, it has already been demonstrated that 1/2 to 3 hour exposures to combinations of SO2 less than 1 ppm and aqueous aerosol droplets less than 0.25 Mum diameter (tagged with Fe less than 90 ng/m3) caused a significant (P=0.01) increase in closing volume relative to vital capacity over that measured under non-exposure conditions. Moreover, the extent of the increase is best described as depending on SO2 concentration and the amount of aerosol deposition (not the concentrations or the depositions of both), suggesting a mechanism for pulmonary irritation in which SO2 is transported together with the aerosol into the lung. The present research will extend the study to additional individuals with the following aims: (1) determine the validity of the initial findings for additional individuals haveing healthy respiratory function; (2) refine the determination of exposure-lung function response, using multiple linear regression and other statistical procedures, in order to gain further understanding of mechanisms for transport of SO2 into small airways and irritation leading to increases in closing volume; (3) carry out similar measurement on human subjects who have some degree of small airway obstruction; (4) estimate the public health consequences of exposures of populations to concentrations of SO2, within current EPA standards, int e presence of fine aqueous aerosol solution particles. In addition, the study will include direct measurement, in each subject, of aerosol deposition in the respiratory tract as a function of particle diameter in the range less than 0.06 to 1 Mum. If significant differences are observed in h patterns of the particle size dependence of fine aerosol deposition for healthy and unhealthy individuals, this result may be made the basis for designing simplified test procedures, for larger populations, to diagnose small airway disease.